The present invention relates generally to a DC-DC converter and more specifically to a current-fed full-bridge DC-DC converter that converts direct-current power into different direct-current power.
A vehicle or a transport machinery (hereinafter referred to collectively as vehicle) that is equipped with an electric motor in addition to an engine for driving the vehicle has been put into practice, such as a hybrid car, a plug-in hybrid vehicle, a hybrid vehicle or a hybrid electric vehicle. There has been recently proposed a plug-in hybrid vehicle whose battery can be charged by system power supply such as household power supply.
Such vehicle is equipped with a traction motor operating at a high voltage. A DC voltage of a battery for driving the vehicle is boosted by a DC-DC converter to another DC voltage that is high enough to drive the traction motor. Thus, the traction motor is operated at the boosted DC voltage. While the vehicle is being slowed down, the traction motor serves as a generator. The voltage of the electric power thus generated by the motor is converted into a battery voltage that is suitable for driving the vehicle, and the power with such converted voltage is used for charging the battery.
The DC-DC converter that converts DC power into different DC power is also used for a solar power system, various electronic devices and appliances such as a television set and a charger for a mobile phone or a computer.
In DC-DC converters, a current-fed full-bridge DC-DC converter has been known. The current-fed full-bridge DC-DC converter is configured so that inputs of an inverter circuit are connected to outputs of a direct-current voltage circuit that is connected in series to a coiled reactor serving as a stationary induction device having an induction effect. Outputs of the inverter circuit are connected to the primary coil of a transformer. The secondary coil of the transformer is connected to a rectifier circuit so that DC power is produced.
The current-fed full-bridge DC-DC converter is characterized in that the input current has a DC continuous waveform with ripples and the input current waveform may be rectified into sinusoidal DC waveform having good power factor and no distortion with a simple converter circuit.
In the DC-DC converter, power conversion is accomplished by switching rapidly the switching elements such as MOSFET (Metal Oxide Semiconductor Field Effect Transistor MOS) or IGBT (Insulated Gate Bipolar Transistor) on and off repeatedly. During the switching operation, the current flowing in the DC-DC converter is changed rapidly, so that overvoltage (or surge voltage) occurs due to the parasitic inductance and capacitance of the DC-DC converter.
In the current-fed full-bridge DC-DC converter, the direct-current circuit including the voltage circuit and the reactor is connected to the transformer through the switching elements of the inverter circuit. There exists a leakage inductance of the transformer in the current path in which the transformer is connected and furthermore the switching elements of the inverter circuit have a parasitic capacitance. During the operation of the current-fed full-bridge DC-DC converter, energy is accumulated in the reactor and the leakage inductance from the current flowing from the current source circuit. If the switching elements of the inverter circuit are turned off rapidly when the current-fed full-bridge DC-DC converter need be stopped, e.g. for the purpose of protection thereof, the energy is accumulated in the reactor and the leakage inductance, but has nowhere to go, so that the parasitic capacitance of the switching elements of the inverter tends to be charged quickly, with the result that overvoltage is generated to the switching elements. If the overvoltage exceeds the withstand voltage of the switching elements, the switching elements may be damaged. To protect the switching elements from such damage, appropriate measures need be taken.
For preventing the switching elements of a current-fed full-bridge DC-DC converter from being damaged by overvoltage generated in the current-fed full-bridge DC-DC converter, Japanese Patent Application Publication 2002-272135 discloses a current-fed full-bridge DC-DC converter to which a snubber circuit is connected for suppressing overvoltage that occurs when each switching element is turned off However, this current-fed full-bridge DC-DC converter is disadvantageous in that the number of parts is increased thereby to bring about an increased cost and also increased loss of circuit during the normal operation of the current-fed full-bridge DC-DC converter.
The present invention is directed to providing a current-fed full-bridge DC-DC converter that has a simple structure and prevents the current-fed full-bridge DC-DC converter from generating an overvoltage in stopping of the current-fed full-bridge DC-DC converter.